Owing to a recent growing demand for development of small vehicles, lightweight vehicles, and the improvement of silence, attempts are being made to manufacture smaller and more lightweight electric auxiliary machine of vehicles, and to make the engine room airtight. On the other hand, there is a growing demand for a high output and a high efficiency for the performance of various apparatuses of the vehicle. Thus the present tendency is to compensate the miniaturization-caused reduction of the output of the electric auxiliary machine of the vehicle provided inside the engine room by rotating them at high speeds.
As examples of rolling bearings for use in the electric auxiliary of the vehicle, a rolling bearing for use in the fan coupling apparatus, a rolling bearing for use in the alternator, a rolling bearing for use in the idler pulley are outline below.
Regarding the fan coupling apparatus, a viscous fluid is sealed inside it. A housing having an air-feeding fan mounted on its peripheral surface is coupled to a rotor directly connected to the engine through a bearing. By utilizing the shear resistance of the viscous fluid which increases and decreases in response to an atmospheric temperature, the fan-coupling apparatus controls the amount of a driving torque transmitted from the engine and the number of rotations of the fan, thereby feeding optimum air corresponding to the temperature of the engine. Therefore the rolling bearing for use in the fan-coupling apparatus is demanded to have a high heat resistance, grease-sealing performance, and durability so that the rolling bearing withstands a high rotational speed change in the range from 1000 rpm to 10000 rpm in dependence on a fluctuation of the temperature of the engine and in addition withstands very strict environment in which it is driven in summer at a high speed not less than 10000 rpm at a high temperature not less than 150° C.
The alternator for use in a car receives the rotation of an engine and generates electricity, thus supplying an electric power to an electric load of the car and has a function of charging a battery. Therefore the rolling bearing for use in the alternator is demanded to have a high heat resistance, grease-sealing performance, and durability so that the rolling bearing withstands a very strict environment in which it is rotated at a high speed not less than 10000 rpm at a high temperature not less than 150° C.
The idler pulley of a car is used as a belt tensioning part for the driving belt transmitting the engine power to the electric auxiliary machine thereof. The idler pulley has the function of a pulley for imparting a tensile force to the driving belt when the distance between rotating shafts is fixed. The idler pulley has another function of an idler used to change the travel direction of the belt or preventing interference with obstacles to thereby reduce the volume of the engine chamber.
Therefore the rolling bearing for use in the idle pulley is demanded to have a high heat resistance, grease-sealing performance, and durability so that the rolling bearing withstands a very strict environment in which it is rotated at a high speed not less than 10000 rpm at a high temperature not less than 150° C.
As a grease composition preferable for the rolling bearing such as the rolling bearing for use in the electric auxiliary machine of a car which is used at high temperature and rotational speed, a grease composition is known. The grease composition contains 0.5 to 10 wt % of amide-based wax having the property of preventing the base oil from being oxidized and having a melting point not less than 80° C. The base oil has a kinematic viscosity of 20 to 150 mm2/second at 40° C. The grease composition contains the urea-based thickener at 5 to 30 wt % for the entire grease composition (see patent document 1).
In recent years, there is a tendency for small motors to be manufactured in industrial machines and for a bearing to be operated at a high surface pressure. In a servo motor, an acceleration and a deceleration to be applied thereto become high in drive—stop. As a result, the bearing has a large amount of slide. At a high temperature, the operation of high-speed drive—sudden deceleration drive—sudden acceleration drive—sudden stop is frequently repeated.
In a manufacturing line of a car, various kinds of industrial robots are used for assembling, welding, painting operations. To decrease a tact time for the improvement of productivity, there is a tendency for the motion speed of the robot to be increased. The operation of the robot is not successive, but intermittent. An increase in the operation speed of the robot causes the rolling bearing for use in a rotational portion to have an increase in the number of switching times of stop—start—operation—stopping operation per time. Each time switching is performed, an acceleration and a deceleration to be applied to the rolling bearing become high. Thereby the rolling bearing generates a large amount of slide.
In recent years, because the use condition for the rolling bearing for use in the electric auxiliary machine of a car, the bearing for use in a motor of an industrial machine, and the bearing for use in a robot has become strict, as described above, a peculiar peeling phenomenon occurs with the rolling surface of the bearing turning into white in its texture.
Unlike peeling which occurs in the inside of the rolling surface owing to metal fatigue, the peculiar peeling phenomenon occurs in the rolling surface at a comparatively shallow portion thereof with the rolling surface turning into white in its texture. That is, the peeling phenomenon is a hydrogen brittleness-caused destruction phenomenon.
As a method of preventing the peculiar peeling phenomenon which occurs in an early stage with the rolling surface turning into white in its texture, a method of adding a passivating agent to a grease composition (see patent document 2) and a method of adding bismuth dithiocarbamate thereto are known (see patent document 3).
In recent years, because the rolling bearing for use in the electric auxiliary machine of a car is used in a strict condition, neither the method of adding the passivating agent to the grease composition nor the method of adding bismuth dithiocarbamate thereto is sufficiently capable of solving the problem of the peeling phenomenon.
A fuel cell system attracts public attention as a new power source of a car or as a dispersion type generating apparatus. A fuel cell has a high output density and is operated at a low temperature. Further a material constructing the cell deteriorates to a low extent. A solid high molecular electrolyte type fuel cell which is actuated easily is effective as a power source of the car or the like.
In the fuel cell system, it is necessary to feed hydrogen and hydrogen rich modified gas serving as fuel and air serving as an oxidizing agent under pressure. To this end, various types of compressed fluid-feeding machines such as a supercharger, an impeller-type compressed fluid-feeding machine, a scroll-type compressed fluid-feeding machine, an iron plate-type compressed fluid-feeding machine, and a screw-type compressed fluid-feeding machine are used.
In the solid high molecular electrolyte type fuel cell, because for power generation, hydrogen serving as fuel and air serving as an oxidizing agent react with each other in a chemical reaction to generate water. In addition, a high molecular film is humidified by a humidifier to function the high molecular film as a solid electrolyte. Thus the solid high molecular electrolyte type fuel cell is constantly kept in a water-containing state. Therefore water is contained in a gas fed under pressure by the compressed fluid-feeding machine. When the water penetrates into the bearing of the compressed fluid-feeding machine, metal contact occurs due to defective lubrication. Therefore as in the case of the rolling bearing for use in the electric auxiliary machine of the car, there is a case in which the hydrogen brittleness-caused peeling phenomenon occurs with the rolling surface of the bearing turning into white in its texture.
Because the use condition for the rolling bearing for use in the fuel cell system has also become strict, the method of adding the passivating agent to the grease composition (see patent document 2) and the method of adding the bismuth dithiocarbamate thereto (see patent document 3) are incapable of solving the problem of the hydrogen brittleness-caused peeling phenomenon. In compliance with a demand for an increase in the generation amount of electricity, the compressed fluid-feeding machine is requested to have high performance and operated at a high speed. Accordingly the rolling bearing is rotated at high speed with a large load applied thereto, which causes the bearing portion to have a high temperature of about 180° C. Thus the rolling bearing is demanded to have excellent heat resistance.
The fan-coupling apparatus, the alternator, and the like which are electric auxiliary machines of a car are used to efficiently utilize the rotational torque of an engine through a one-way clutch which is connected to an engine output shaft only when the engine is in a predetermined output state.
Representative electric auxiliary machines for the car which operate through the one-way clutch includes the fan-coupling apparatus which feeds air most suitably in correspondence with the temperature of the engine and the alternator which generates electricity by receiving a rotation of the engine, supplies an electric power to an electric load of a vehicle, and charges a battery. These electric auxiliary machines are used to efficiently utilize the rotational torque of the engine through the one-way clutch to be connected to the engine output shaft only when the engine is in a predetermined output state. The one-way clutch mounted on the electric auxiliary machine for the car is frequently connected to and disconnected from the engine output shaft. When the one-way clutch is connected to the engine output shaft, it has a high rotational speed. Consequently a high load is applied to the one-way clutch, and it generates a large amount of heat and vibration. The use condition for the one-way clutch has become strict, as the car has come to have high performance and output.
When the use condition for the one-way clutch has become strict, the hydrogen brittleness-caused peeling phenomenon is liable to occur on rolling contact surfaces of balls of the one-way clutch, rolling surfaces of an outer ring for the clutch, and rolling surfaces of an inner ring for the clutch connected to a shaft on which the one-way clutch is mounted with these rolling contact surfaces and the contact surfaces turning into white in the textures thereof when a roller clutch overruns. In addition, when the one-way clutch is locked, there is a possibility that contact portions between these rolling contact surfaces and rolling surfaces are liable to fret. Therefore grease which is enclosed in a space where the rollers are mounted is desired to prevent the generation of the peeling and fretting phenomena.
The electric auxiliary machine on which the one-way clutch has been mounted is frequently installed on a lower portion of the engine room. Thus while a car is travelling, rainwater is liable to penetrate into the one-way clutch. When the rainwater penetrates into the space in which the rollers of the one-way clutch are mounted, the rolling contact surfaces of the rollers of the one-way clutch, the rolling surfaces of the outer ring for the clutch, and the rolling surfaces of the inner ring for the clutch connected to the shaft on which the one-way clutch is mounted are liable to corrode. Therefore the grease which is enclosed in the space in which the rollers are mounted is required to have rust-preventing performance superior to that of grease which is used in other portions.
In the conventional one-way clutch for use in the alternator, the one using grease containing ether oil as its base oil (see patent document 4), the one using grease whose viscosity pressure coefficient is not less than a predetermined value (see patent document 5), and the one using grease containing synthetic oil whose kinematic viscosity at 40° C. is not more than 60 mm2/second as its base oil (see patent document 6) are known.
But in the patent document 4, the one-way clutch using the grease containing the ether oil as its base oil has a problem that the one-way clutch has an insufficient low frictional wear property when the one-way clutch is in an overrun state. In the patent document 5, the use of the grease whose viscosity pressure coefficient is not less than the predetermined value is effective for securely realizing a locked state, but has a problem that the grease is incapable of sufficiently restraining wear in the overrun state. In the patent document 6, the grease containing the synthetic oil whose kinematic viscosity is low as its base oil has an insufficient heat resistance. Thus it is difficult to use the grease for a long time.
Patent document 1: Japanese Patent Application Laid-Open No. 2003-105366
Patent document 2: Japanese Patent Application Laid-Open No. 3-210394
Patent document 3: Japanese Patent Application Laid-Open No. 2005-42102
Patent document 4: Japanese Patent Application Laid-Open No. 11-82688
Patent document 5: Japanese Patent Application Laid-Open No. 2000-234638
Patent document 6: Japanese Patent Application Laid-Open No. 2000-253620